1. Field of the Invention
The present invention relates to a magnetic bearing arrangement for an artificial satellite, and more particularly, to a magnetic bearing for a rotor which has a rotating speed and an axis of rotation adjusted in accordance with a change in the attitude of the artificial satellite in order to control it.
2. Description of the Prior Art
The attitude of an artificial satellite is controlled as generally illustrated in FIG. 1. The angles of rotation of an artificial satellite about three axes, X, Y and Z are called its attitude angles designated as .phi. (roll), .theta. (pitch), and .psi. (yaw),respectively. A flywheel usually has an axis of rotation which is in parallel to the axis Y, and rotates at a constant speed. The flywheel is supported by a magnetic bearing so that its axis of rotation may be slightly inclined toward the X or Z axis at an angle .alpha. or .beta., respectively. Accordingly, it is possible to control the angle .psi. about the yaw axis by varying the angle .alpha., the angle .phi. about the roll axis by varying the angle .beta., and the angle .theta. about the pitch axis by varying the rotating speed of the flywheel.
The attitude control for the satellite may be performed by an apparatus as shown in FIG. 2. The apparatus includes a rotor 1, an axial displacement sensor 2, an electromagnet 3 providing an attractive force controlled in accordance with an output signal from the sensor 2 to move the rotor 1 to a fixed axial position, a radial displacement sensor 4 for detecting the radial displacement of the rotor 1, an electromagnet 5 providing an attractive force controlled in accordance with an output signal from the sensor 4 to move the rotor 1 to a fixed radial position, a stator 6, and a motor 7 for driving the rotor 1. A pair of sensors 4 and a pair of electromagnets 5 are provided to enable the rotor 1 to be restored to its original position when its axis of rotation has been inclined.
Thus, the six degrees of freedom of the rotor 1, with the exception of its rotatory displacement about its axis of roataion, are completely controlled by the sensor and electromagnet combinations. Therefore, the rotor 1 can be rotated without being brought into any contact with the stator 6. The apparatus is, thus, advantageous in various respects as a system for controlling the attitude of an artificial satellite.
Five sensor and electromagnet combinations are, however, required for effecting the radial control of the rotor 1 along the x and y axes which are prependicular to each other and to the axis of rotation of the rotor 1, designated as the z axis, its axial control along the z axis, and the control of inclination of its axis of rotation, i.e., its angular displacement about the x or y axis.
Another example of a system for actively controlling five modes of operation, namely an axial translation, two radial translations and two tilting motions, is found in a paper by R. S. Sindlinger entitled "Magnetic Bearing Momentum Wheels with Vernier Gimballing Capability for 3-Axis Active Attitude Control and Energy Storage", appearing in the 1976 IFAC Symposium Automatic Control in Space.
In the magnetic bearing wheels as hereinabove described, the radial displacement electromagnets do not produce a magnetic flux which is uniform along the circumference of the the wheel. Therefore, an eddy current appears on the rotor and creates resistance to its rotation. Since all the control is effected by an electromagnetic force, the apparatus consumes a lot of power even during normal operation, and requires a complicated control circuit.
In another publication entitled "Satellite Flywheels with Magnetic Bearings and Passive Radial Centering" by P. C. Poubeau, appearing in the AIAA Journal of Spacecraft, vol 17, No. 2, Mar.-Apr., 1980, the use of passive permanent magnet radial bearings is disclosed. However, the stator is of one-piece with the permanent magnets being in the form of continuous annular rings. The radial bearing, the axial bearing and the radial dampers are separated from each other and spaced along the axis of the wheel. The radial damper does not provide for the positive control of the inclination of the axis of rotation.